Method of making spheroidal high explosive particles having microholes dispersed throughout

ABSTRACT

CRYSTALLINE HIGH EXPLOSIVE ARE FORMED INTO FINELY-DIVIDED SPHEROIDAL PARTICLES BY MIXING SEPARATE STREAMS OF A SOLUTION OF THE EXPLOSIVE DISSOLVED IN SOLVENT WITH AN INERT NONSOLVENT SO AS TO OBTAIN NONLAMINAR FLOW OF THE STREAMS BY APPLYING PRESSURE AGAINST THE FLOW OF THE NONSOLVENT STREAM SO AS TO DIVERGE SAID STREAM AS IT CONTACTS THE SOLUTION OF EXPLOSIVE IN SOLVENT AND VIOLENTLY AGITATING THE COMBINED COMBINED STREAM TO RAPIDLY PRECIPITATE THE EXPLOSIVE FROM SOLUTION IN THE FORM OF SPHEROIDAL PARTICLES PERMEATED WITH MICROHOLES. THE RESULTING HIGH EXPLOSIVE CONSIST ESSENTIALLY OF SPHERIODAL PARTICLES, THE PARTICLES CONSIST OF AGGLOMERATED CRYSTALLITES THAT ARE SUBSTANTIALLY SPHERICAL. THE SPHEROIDAL PARTICLES HAVE MICROHOLES THAT HAVE AN AVERAGE DIAMETER OF ABOUT FROM 0.1-02 MICRON DISPERSED THROUGHOUT THE PARTICLES IN LARGE NUMBER.

METHOD OF MAKING SPHEROIDAL HIGH EXPLOSIVE PARTICLES HAVING MICROHOLES DISPERSED THROUGHOUT Filed Aug. 13. 1971 Aug. 21, 1973 c D. FORREST ET AL 3,754,061

FiG-l INVENTORS CHARLES D. FORREST CECIL e. MILLER, JR.

ATTORNEY United States Patent O 3,754,061 METHOD OF MAKING SPHEROIDAL HIGH EX- PLOSIVE PARTICLES HAVING MICROHOLES DISPERSED THROUGHOUT Charles D. Forrest, Martinsburg, W. Va., and Cecil G. Miller, Jr., Hagerstown, Md., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del.

Filed Aug. 13,'1971, Ser. No. 171,429 Int. Cl. C06h 21/02 US. Cl. 264-3 ll) 16 Claims ABSTRACT OF THE DISCLOSURE Crystalline high explosives are formed into finely-divided spheroidal particles by mixing separate streams of a solution of the explosive dissolved in solvent with an inert nonsolvent so as to obtain nonlaminar flow of the streams by applying pressure against the flow of the nonsolvent stream so as to diverge said stream as it contacts the solution of explosive in solvent and violently agitating the combined stream to rapidly precipitate the eX- plosive from solution in the form of spheroidal particles permeated with microholes. The resulting high explosive consists essentially of spheriodal particles, the particles consist of agglomerated crystallites that are substantially spherical. The spheroidal particles have microholes that have an average diameter of about from 0.1-0.2 micron dispersed throughout the particles in large number.

BACKGROUND OF THE INVENTION This invention relates to a method for making novel spheroidal particles of high explosives permeated with microholes.

Normally crystalline high explosives have been treated by various techniques to reduce their particle size. It has been thought that the particular particle size of explosives has a pronounced effect on its performance, and that generally the smaller the particles the more sensitive the explosive is to reliable propagation sensitivity. Heretofore particles of high explosive have been prepared by dissolving the explosive in a solvent that is inert to the explosive and mixing the solvent with a liquid that is a nonsolvent for the explosive and is miscible with the solvent, or drowning the solution of explosive in the nonsolvent precipitating agent. Further, various modifications of these processes are known wherein, for example, additional nonsolvent is added to the turbulent mixture in Order to produce fine crystals of high explosive, as described, for example, in British Pat. 988,122 published Apr. 7, 1965. Such procedures have employed eductors or jet nozzles, as illustrated in Canadian Pat. 533,487, for mixing one stream containing explosive dissolved in solvent and the other stream containing the nonsolvent precipitating agent. Such procedures produce small particles of high explosive, but the finely-divided explosives made by such methods do not consistently propagate detonations and are unreliable and erratic, especially when used in compositions wherein the particulate explosive is incorporated in a binder and the final product is formed into thin sheets or small diameter explosive cord. Therefore, a need exists for high explosives that can be used in such thin sheets or small diameter cords and still consistently propagate detonation and that exhibit a high order of sensitivity.

SUMMARY OF THE INVENTION It has now been discovered that finely-divided normally crystalline high explosives can be prepared that will consistently propagate detonation when said explosive is incorporated in a binder and the final product is formed into thin sheets of, for example, thicknesses of 0.025 inch and small diameter, e.g., about a millimeter, detonating 3,754,061 Patented Aug. 21, 1973 r... CC

cord. The normally crystalline high explosive is converted into finely-divided spheroidal particles that are permeated by microholes. The process for making such explosive comprises mixing separate streams of a solution of the explosive dissolved in an inert solvent and of an inert nonsolvent miscible with the solvent in such a manner so as to obtain nonlaminar flow of the streams by applying pressure against the flow of the nonsolvent stream so as to diverge said stream as it contacts the solution of explosive in solvent and thereby entrap the solution of explosive in solvent in minute droplets in the nonsolvent, violently agitating the resulting combined stream so as to subsequently rapidly precipitate the explosive from solution in the form of spheroidal particles permeated with microholes. It is critical to the successful operation of the process that the stream of explosive dissolved in inert solvent and the stream of inert nonsolvent are intimately mixed so that laminar flow of the streams does not occur. Nonlaminar flow of the streams coupled with violent agitation of the combined stream so as to obtain rapid precipitation of the explosive is necessary to obtain particles of explosive that are spheroidal and have microholes throughout.

The mixing of the explosive dissolved in the inert solvent and inert nonsolvent is usually conducted in a confined mixing chamber. Conveniently, the process can be conducted in a modified eductor so as to provide nonlaminar flow of the streams together with violent agitation of the combined stream resulting in rapid precipitation of the explosive. Pressure of about from 1 to 30 pounds per square inch gauge, usually 2 to 5 pounds per square inch gauge, are applied against the flow of the nonsolvent stream to assure conditions that result in nonlaminar flow of the streams. Accordingly, the apparatus discharges against a pressure. Such pressure causes the nonsolvent stream to diverge or disperse, that is fan out, subsantially instantaneously as it enters the mixing chamber and contacts the solution of explosive in solvent thus causing rapid and intimate mixing of the streams. Conveniently, the nonsolvent stream is pumped at pressures of about from 50 to 150, usually to 125, pounds per square inch guage. Precipitation of the explosive from the time it is contacted with nonsolvent is rapid. Generally, the solution of explosive and nonexplosive are mixed for at least 2 milliseconds and no more than about 6 milliseconds at which time substantially complete precipitation has occurred. Rapid precipitation is necessary to obtain explosives in which all particles are spheroidal permeated with microholes.

The process of the present invention results in a novel high explosive that is highly sensitive and propagates detonations when said explosive is incorporated in a binder and formed into thin sheets or very small diameter explosive cord. The novel explosives are pentaerythritol tetranitrate, cyclotrimethylene trinitramine, trinitrotoluene and cyclotetramethylene tetranitramine. These finelydivided high explosives can be characterized as consisting essentially of spheroidal particles, said particles consisting of agglomerated crystallites of the explosive wherein said crystallites are substantially spherical, the crystallites of pentaerythritol tetranitrate, cyclotrimethylene trinitramine and trinitrotoluene have an average diameter of about from 0.1-0.2 micron and the crystallites of cyclotetramethylene tetranitramine have an average diameter of about from 0.03-0.04 micron, said spheroidal particles of high explosive having microholes dispersed throughout said particles. Substantially all the particles of the explosive have diameters between about from 2 to 150 microns. Usually, of the particles of pentaerythritol tetranitrate have a diameter of at least about 2 microns and substantially all the particles are no greater than 44 microns. Since these particles are not spherical. the measurements as to particle size distribution were made by micromerigraph and particle shape was determined by optical microscopy. The number of microholes in the spheroidal particles made by the process of this invention is, generally, larger by one or two orders of magnitude than the number of microholes found in the usually rodor needle-like particles of explosives made by prior art processes. The concentration of microholes in the spheroidal particles made by the present process is at least equivalent to a concentration of above about 500, usually above 1000, microholes based on a spheroidal particle having a diameter of 6 microns. Accordingly, particles of other diameters will have proportional microhole concentrations, for example, spheroidal particles having diameters of 20 microns will have above about 8000 microholes.

Important distinguishing characteristics of these novel high explosives is that the particles have a spheroidal shape, the microholes are dispersed throughout the particles and the stated average diameter of the crystallites and microholes. It is necessary that the particles be spheroidal, as distinguished from particles that are rod-shaped or needle-like. Secondly, it is essential that the particles of high explosive contain microholes that have an average diameter of about from 0.1-0.2 micron dispersed throughout the particles. These microholes are true cavities and are not interconnecting channels.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates a schematic diagram for carrying out the invention, and

FIG. 2 is a sectional view of a preferred mixing device.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. 1 which illustrates a preferred embodiment of the invention, 1 represents a reservoir for the solution of normally crystalline high explosive. 2 is a reservoir for the nonsolvent for the explosive composition. 3 represents the mixing device and 7 the back pressure assembly, i.e. means for applying pressure against the flow of the nonsolvent stream, both fully detailed in FIG. 2. 4 is a pump assembly for the nonsolvent including control valves to regulate the pressure and flow, 5 represents a nonsolvent transport pipe from pump 4 to mixing device 3, 6 is an inlet tube from reservoir 1 to mixing device 3. The mixing device discharges directly into the back pressure assembly 7 from which the total efliuent flows via transport means 8 to a recovery zone where the solid spheroidal particles of explosive are separated by conventional means, for example, filtration, from the liquid, said liquid portion being transported to a solvent-nonsolvent separating zone for possible reuse in the process.

In FIG. 2, mixing device 3 is provided with inlet tube 9 having a nozzle 10 which is positioned in the upper portion of inlet tube 6. The mixing chamber 11 has a convergence zone 13 forward of nozzle 10 that assists in drawing the stream in inlet tube 6 into the mixing chamber by suction. A throat-like portion 12 of the mixing chamber communicates with difi'user 14. Dilfuser 14 communicates directly with back pressure assembly 7; said back pressure assembly comprising tube 15 containing curved sheet-like elements 16 that extend in series longitudinally within said tube, such device being more fully described in detail in Pat. 3,286,992 to D. D. Armeniades et al. The particular mixing device illustrated in the drawing is a modified eductor-type device. However, it is important to remember that in order to obtain intimate and instantaneous mixing of the streams and prevent laminar flow of the streams, pressure must be applied against the flow of the nonsolvent stream issuing out of nozzle 10 so as to diverge or fan out the stream. The tip or mouth of the nonsolvent feed nozzle 10 is positioned such that the diameter of the divergent nonsolvent stream at the inlet to the throat is equal to the diameter of the throat.

The operation of the present system is as follows: Nonsolvent, e.g. water, flows under pressure from reservoir 2 through pump 4 and pipe 5 to mixing device 3 entering through nozzle 10. Pressure is applied against the flow of the nonsolvent stream by means of back pressure assembly 7 to diverge or disperse the stream. The solution of explosive dissolved in solvent entering through inlet tube 6 is intimately and rapidly mixed with nonsolvent in mixing chamber 11, especially throat 12. Mixing and precipitation continue as the combined stream flows through throat 12 to diffuser 14 at which time precipitation is substantially complete. The solution of explosive and nonsolvent precipitating agent are usually mixed for no more than about 6 milliseconds at which time substantially complete precipitation of the explosive has occurred. The material flows through back pressure assembly 7 to a recovery zone wherein the spheroidal particles of explosive containing microholes are separated by, for example, filtration, from the liquid and subsequently dried. The liquid solvent-nonsolvent is subsequently separated by distillation or other conventional means.

Pressure is applied against the flow of the nonsolvent stream. Such pressure, referred to as back pressure, does among other things, cause intimate contact of the streams for rapid precipitation. For example, in eductor type apparatuses back pressure has the effect of creating a divergent fanned out nonsolvent stream. This divergent stream provides intimate and substantially instantaneous mixing of the stream of explosive dissolved in inert solvent and the stream of inert nonsolvent. The amount of back pressure applied to the nonsolvent stream in the mixer will vary somewhat depending upon the design of the mixing apparatus, e.g., eductor, and the dimensions of the apparatus and the pressures of the inert nonsolvent, e.g., water. However, the pressure difference between the motive fluid, i.e. nonsolvent, and back pressure, taking into account the design of the particular apparatus, is usually so regulated that the combined stream will be mixed and the explosive substantially fully precipitated in no more than about 6 milliseconds. Generally, intimate mixing and rapid precipitation occur in about from 2 to 6 milliseconds. Preferably, the amount of back pressure applied against the nonsolvent to produce spheroidal particles having microholes throughout is from about 2-5 pounds per square inch gauge and the nonsolvent stream is pumped at a pressure of about from 75 to pounds per square inch gauge.

A number of means can be used to apply pressure against the flow of the nonsolvent stream. For example, such back pressure can be generated by a restriction placed in the discharge line such as a reduced orifice or a valve attached to the end of diffuser 14. One especially suitable means for obtaining back pressure involves the use of a hollow cylindrical tube having a plurality of curved sheetlike elements extending longitudinally within the tube, as illustrated in the drawing.

The novel product produced by the process of the present invention can be used in the same manner and for the same purpose as other high explosives, however these products exhibit characteristics not found in explosives made by prior art processes. The explosives can be characterized as containing only spheroidal-shaped particles. The particles consist of agglomerated crystallites of the explosive and the crystallites are substantially spherical having a particle average size diameter. The explosives contain microholes that have an average diameter of about from 0.1-0.2 micron that are dispersed throughout said particles. Generally, the porosity of the particles is such that their density is less than about 97% of the theoretical product density. Ninety-five percent of the particles have a diameter of at least 2 microns and substantially all the particles have diameters of about from 2 to 44, usually 22, microns and the mass median particle diameter is about from 5-7 microns.

As mentioned above, the spherical particles of explosive made by the process of this invention contain microholes. The loose packing of the crystallites constituting the particle leave small microholes between the crystallites. Such microholes are not channels but are true cavities that lie at all depths throughout the particle. Loose random packing of the crystallites occur following the process of this invention and such packing results in the production of a high concentration of microholes throughout the particle. All the particles made by the present process exhibit this high concentration of microholes.

It is believed that the propagation sensitivity of the particles is enhanced by the presence of microholes. Air entrapped within the microholes act as hot spot initiation centers when the gas is adiabatically compressed during the initiation stages of explosion and, further, it is thought that the microholes provides a structure for the Munroe jet efiect.

Representative crystalline high explosives which can be prepared in the form of spheroidal particles permeated with microholes include organic nitrates such as pentaerythritol tetranitrate (PETN), and nitromannite, nitramines such as cyclotrimethylene trinitramine (RDX), cyclotetramethylene tetranitroamine (HMX), tetryl, ethylene dinitramine, and aromatic nitro compounds such as trinitrotoluene (TNT).

Solvents used in the process are those which dissolve the high explosive, are inert to the explosive, and are miscible with the nonsolvent for the explosive. Representative solvents that can be used are ketones such as acetone, methylethyl ketone, cyclopentanone, and cyclohexanone; esters such as methyl acetate, ethyl acetate and B-ethoxy-ethyl acetate; chlorinated aromatic hydrocarbons such as chlorobenzene; nitrated hydrocarbons such as nitrobenzene and nitroethane; nitriles such as acetonitrile; and amides such as dimethyl formamide. Acetone is especially preferred because it is inexpensive, a good solvent for the explosives and is readily miscible with water yet is readily separated by distillation. Sufficient solvent is used to completely dissolve all the explosive to be precipitated as small spheroidal particles containing microvoids.

Preferably the concentration of the explosive in the solvent should be high for economic reasons. In a PETN- acetone system at temperatures of about from 20 to 60 C., the PETN preferably will constitute from about to 40% by weight of the solution. Generally, the temperature of the explosive-solvent stream is from about 35- 60 C.

Any nonsolvent for the explosive which is miscible with the solvent may be employed. Representative nonsolvents that can be used in the process are ethers such as methylethyl ether, diethyl ether, ethylpropyl ether and vinyl ether; alcohols such as methanol, ethanol, isopropanol and isobutanol; aromatic hydrocarbons such as benzene and toluene; and chlorinated aliphatic hydrocarbons such as ethylene dichloride, trichloroethylene, trichloroethane, carbon tetrachloride, and chloroform. The preferred nonsolvent is water, primarily because of its low cost. In general, flow rates of the nonsolvent are from about 6 to 12 gallons per minute. The pressure of the nonsolvent entering the mixing chamber generally is of the order of 50 to 150 pounds per square inch gauge.

The following examples further illustrate the present invention.

Example 1 An assembly is set up as illustrated in FIG. 1. Filtered water (15 C.) is pumped through the eductor nozzle (0.17 inch ID.) at a pressure of about 120 pounds per square inch gauge at a rate of about 11 /2 gallons per minute. PETN is dissolved in acetone to form a 17% solution at about 35-40 C. and is fed through a filter in the inlet tube 4 inch ID.) to the mixing chamber of the eductor at a rate of about 6 gallons per minute. A pressure, so-called back pressure, of about 4 pounds per square inch gauge is applied against the flow of the nonsolvent stream issuing out of the eductor nozzle causing it to diverge or fan out. Consequently, the separate streams of explosive in solution and of nonsolvent are turbulently mixed so as to obtain nonlaminar flow of the streams. Violent agitation of the combined stream occurs. Subsequently the nonsolvent dilutes the solvent and causes precipitation. The solution of explosive and nonsolvent were mixed for about 4 milliseconds at which time substantially complete precipitation occured. The precipitated PETN and the liquid solvent-nonsolvent flowed through the back pressure assembly and was separated by filtration.

Microscopic examination of the solid PETN revealed that the particles were substantially all spheroidal and consisted of agglomerated crystallites. The crystallites were spherical and had an average diameter of about 0.15 micron. Microholes were present in the particles, due to the fact that the crystallites were loosely packed, and had an average diameter of about 0.2 micron. The microholes were dispersed throughout the particle. The particles had a mass median diameter of about 6 microns and about 98% of the particles had diameters of less than 20 microns. The density of the particles was 1.70 grams/ cc. which is about 96% of the theoretical density. Tests indicated that the particles had a concentration of more than 1000 microholes in a spheroidal particle having a 6- micron diameter.

Samples of the resultant PETN were formed into thin sheets of explosives having a thickness of 0.025 inch. The sheets contained 63% PETN, 29% acetyltributyl citrate and 8% nitrocellulose. The explosive sheets consistently propagated detonation along their entire length at velocities of about 6900 meters per second.

Example 2 The procedure described above in Example 1 is repeated except that RDX is substituted for the PETN. Dry RDX is dissolved in acetone to form at 50 C. a 7% solution of the explosive in solvent. The solution is fed to the eductor at about 6 gallons per minute where it is contacted with the nonsolvent stream of water. A pressure of about 3 pounds per sequare inch gauge is applied against the flow of the Water issuing out of the eductor nozzle thus causing the stream of water to disperse or diverge.

Examination of the dry RDX particles shows that substantially all particles have diameters that are less than about 20 microns, the average being about 5 microns; the particles consist of randomly agglomerated crystallites that have diameters of about from 0.l30.15 micron; and microholes of the particles were measured to be between about 0.15-0.21 micron, averaging 0.18 micron.

Example 3 The procedure described above in Example 1 was repeated except I-IMX was substituted for PETN.

Examination of the particles shows that the crystallites had diameters of about from 0.03-0.04 micron and microholes having an average diameter of about 0.17 micron.

Example 4 The procedure described in Example 1 was repeated except that a 20% solution of trinitrotoluene and acetone at 30 C. was substituted for the PETN.

Examination of the product showed that substantially all the particles had diameters that were less than about 30 microns.

We claim:

1. A process for converting normally crystalline high explosive into finely-divided spheroidal particles which comprises mixing separate streams of a solution of the explosive dissolved in an inert solvent and of an inert nonsolvent miscible with the solvent under conditions of nonlaminar flow by diverging said nonsolvent stream by applying pressure against the flow thereof as it contacts the solution of explosive in solvent, entrapping the solution of explosive in solvent in minute droplets in the nonsolvent, violently agitating the resulting combined stream and rapidly precipitating the explosive from solution in the form of spheroidal particles permeated with microholes.

2. A process of claim 1 wherein the crystalline high explosive is pentaerythritol tetranitrate.

3. A process of claim 1 wherein the crystalline high explosive is cyclotrimethylene trinitramine.

4. A process of claim 1 wherein the crystalline high explosive is trinitrotoluene.

5. A process of claim 1 wherein the crystalline high explosive is cyclotetrarnethylene tetranitramine.

6. A process of claim 2 wherein the solvent is acetone.

7. A process of claim 2 wherein the pressure applied against the flow of nonsolvent stream is about from 1 to 30 pounds per square inch gauge.

8. A process of claim 7 wherein the pressure applied against the flow of nonsolvent stream is about from 2 t 5 pounds per square inch gauge.

9. A process of claim 7 wherein the nonsolvent stream is pumped at pressures of about from 50 to 150 pounds per square inch.

10. A process of claim 8 wherein the nonsolvent stream is pumped at a pressure of about from 75 to 125 pounds per square inch gauge.

11. A process of claim 10 wherein the solution of explosive and nonsolvent are mixed for at least 2 milliseconds and no more than about 6 milliseconds at which time substantially complete precipitation has occurred.

12. A process for converting normally crystalline pentaerythritol tetranitrate into finely-divided spheroidal particles which comprises mixing separate streams of a solution of pentaerythritol tetranitrate dissolved in acetone and of Water under conditions of nonlaminar flow by substantially instantaneously diverging said water stream as it contacts the solution of explosive in acetone by applying pressure against the flow of the water, entrapping the solution of pentaerythritol tetram'trate in acetone in minute droplets in the Water, violently agitating the resulting combined stream and rapidly precipitating the explosive from solution in the form of spheroidal particles permeated with microholes.

13. A process of claim 12 wherein the stream of water is pumped at pressures of about from to 125 pounds per square inch gauge.

14. A process of claim 13 wherein the pressure applied against the flow of the stream of water is about from 2 to 5 pounds per square inch gauge.

15. A process of claim 13 wherein the acetone solution of pentaerythritol tetranitrate and water are mixed for about from 2 to 6 milliseconds at which time substantially complete precipitation has occurred.

16. A process of claim 13 wherein the explosive is dried.

References Cited UNITED STATES PATENTS 1,106,087 8/1914 Du Pont 264 3 C 2,329,575 9/1943 Allison et a1. 2643 C X 2,715,574 8/1955 COX 2643 E X STEPHEN J. LECHERT, 111., Primary Examiner US. Cl. X.R. 1492, 92, 93, 

